Probe fed pillbox antenna with pattern shaping pins at aperture



Dec. 288, 1965 F. H. YANG PROBE FED PILLBOX ANTENNA WITH PATTERN SHAPINGPINS AT APERTURE 2 Sheets-Sheet 1 Filed Aug. 17, 1962 TH IHIIII ll llllll I Z4 jmaw m di 221i;

Dec. 28, 1965 R. F. H. YANG PROBE FED PILLBOX ANTENNA WITH PATTERNSHAPING PINS AT APERTURE 2 Sheets-Sheet 2 Filed Aug. 17, 1962 UnitedStates Patent 3,226,722 FED FILE-3G2; ANTENNA WITH EATTERN SHAllNG ENSAT AhElRTURlE Richard F, H. Yang, (Erland Park, ill, assignor to AndrewCorporation, @rland Park, llli., a corporation of Ellinois Filed Aug.17, 1962, Ser. No. 217,762 ill Claims. (Cl. 343-480) This inventionrelates to directive antennas, and more specifically to the type ofantenna having a radiating feed at the focus of a parabolic refiector.The invention is of particular utility in connection with the so-calledpillbox antenna, characterized by a cavity having two parallel wallswith a rear wall extending perpendicularly to, and connecting, theparallel walls, and describing a parabola having its focus on the frontportion of this axis, the front portion being substantially open andforming the mouth. In such antennas, the feed is disposed at the focus,and the spacing between the parallel plane walls is normallysuiiicientiy small so as to permit the propagation of only a single modewith the polarization direction of the feed.

One important use of highly directional antennas is in connection withradar, and the horizontally oriented pill-box antenna, suitably mountedfor rotation is frequently used in marine radar, the relation of thesharpness of the directivity in the horizontal plane compared to that inthe vertical plane being particularly advantageous in obtaining theradar intelligence there res ed, azimuth and distance being essentiallythe only important requirement in such a system, and this type ofantenna being accordingly well-suited for the purpose, requiring onlyrotation about a vertical axis and being compact and convenient,particularly in the small sizes with which very high directivity may beobtained at frequencies of the order of thousands of megacycles. At suchfrequencies, parabolas of very large aperture (in terms of wavelength)may be employed in the very shallow cavities thus required, forming arelatively plate-like overall structure except for the horn flaring inthe vertical plane, which is provided to give reasonable directionalityin that plane to prevent waste of the radiated energy at large upwardand downward angles.

One limitation on the use of high-frequency radar in relativelyinexpensive systems suitable for use on small ships, and particularly onpleasure craft, is the precision of fabrication required Where fullyreliable sharp horizontal patterns are to be obtained. In theory, itwould appear that the ability to employ an aperture of a width of manywavelengths should make the achievement of high horizontal directivityrelatively simple. In practice, however, although a sharp main beam maybe obtained without too much difficulty, the employment of designs andconstrucion practices of a cost compatible with such uses has, prior tothe present invention, encountered great difiiculty with the additionalproduction of unwanted and spurious sidelo'oes the radiation pattern,which, although of substantially smaller magnitude than the main beam,create serious ambiguities in the radar information, particularly inview of the vast differences in reflectivity of various kinds of objectswhich must be detected by the system, simple and easily fabricated typesof construction of such antennas heretofore available having sidelobesof such amplitude that a single detected object might appear in theradar pattern with a multiude of ghosts in which second and thirdobjects of lesser reflectivity might be completely obscured. Thisproblem of spurious sidelobes can well render the entire marine radarsystem useless for its intended purpose, irrespective of sharpness ofthe main beam, particularly in the simple types of manners in which thereflected signals must be handled in the electronic systems which areeconomically practical for such uses, computational features of the typedesigned to distinguish between sidelobe and true images by storing andusing information from the entire radar sweep to distinguish betweenspurious and desired images being completely impractical in general.

The pattern difficulties just mentioned are found to flow from a numberof sources, in general connected with the required limits imposed byeconomics, prior to the present invention. At frequencies of the orderof a number of gigacycles, with wavelengths of the order of an inch orso, small dimensional irregularities anywhere in the cavity, of anabsolute magnitude which would be negligible at lower frequencies,become of suflicient magnitude with respect to the wavelength that thereresult departures from theoretical operation produced by interferencepatterns and similar phenomena due to variations of phase relations andaxial symmetry in the emitted wave-front from the theoretical ideal, dueto differences in phase velocity within the cavity and reflection at themouth resulting from deviations from complete fiat parallelism of thewalls and similar phenomena. Such difficulties are multiplied where, foreconomic reasons, the cavity and its associated horn are formed frommaterials of sufficient thinness and flexibility to permit fabricationof the parabolic surface and the horn by bending and similar techniques,the employment of such techniques as machining from solid blocks beingclearly prohibitive. The use of a large aperture for purposes ofnarrowing the main beam involves, in such an antenna, the employment oflarge sheets for the top and bottom parallel cavity surfaces, withconsequent dimensional instability in constructions heretofore use.

Another source of difficulty which has heretofore been encountered ineconomically practical constructions is the matter of direct radiationfrom the feed, which of course does not have the benefit of the focusingeffect of the reflector. For economic reasons, the feed employed must beof a very simple type. The use of such complexities as a horn waveguidetermination disposed within the cavity for illumination of the parabolicreflector wall is economically impractical, the difficulties ofconstruction of such a feed for uniform illumination, coupled with thenecessity of rotating waveguide joints, etc., thus required, beingprohibitive, so that the cavity is normally excited by a simpleprobe-type radiator at the focus, such a probe in itself inherentlyproducing a circularly syi metrical pattern, and the pill-box beingrotated about the focus as an axis in the radar sweep. In order toreduce the forward radiation directly from the probe, which is of courseunfocussed, a reflector element may be secured immediately forward ofthe radiating probe in the cavity. It is found, however, that if such areflector is made of suflicient width to attempt to shield the directradiation from the entire forward solid angle, whether straight orcurved, the theoretical pattern is again destroyed by the improper phaseand directional properties of reflections to the parabolic surface whichcome from the lateral portions of the reflector.

it has been found that simple and expensive marine radar antennas of thepill-box type were not completely practical as regards freedom fromobjectionable sidelobes, and particularly in the region of the mainbeam, prior to the .present invention, for one or more of the reasonsstated above. The present invention flows from a study of the possibleor probable causes of inadequate sidelobe performance in this type ofantenna, and the present invention provides a simple and inexpensive,but highly effective, construction for such antennas as regardsdesirable pattern characteristics. (it will be understood that thepatterns and structural features herein discussed, including terms suchas radiate, exciter,, etc., include both transmission and reception, inaccordance with the usual practice flowing from reciprocity.) It isexperimentally found that the construction hereinafter to be describedproduces a great reduction in sidelobe amplitude as compared withpreviously known constructions, with no reduction in sharpness oramplitude of the main forward beam. In the present construction, thereare disposed across the mouth of the cavity, inwardly adjacent to thecommencement of the flaring which forms the horn, a large number ofparallel mutually spaced conducting pins or posts extendingperpendicularly between the parallel walls and spaced by a distance offrom one-half to one wavelength, the lateral dimension being very shortcompared to a wavelength, i.e., substantially less than a quarterwavelength. This linear array of .posts or pins (what is known of theexact theory of operation would indicate that thin plates extending inthe direction of focus might be equally good electrically, although muchmore expensive for comparable performance, for reasons which will laterbe seen) extends across the entire mouth and is only very slightly (lessthan a wavelength) forward of the parabolic focus at which the excitingprobe is located, so that the spacing between the pins, althoughappearing as of the order of magnitude of a wavelength in the desireddirection of propagation, i.e., in the direction in which radiationsfrom the omnidirectionally radiating exciting conductor are reflected bythe parabolic surface, appear effectively as a solid wall of conductorfor radiations from the position of the exciting conductor at anglesbeyond the angles subtended by a simple screw-type reflector immediatelyforward of the exciting conductor, thus shielding against directradiations and radiations reflected from the sidewalls of the hornportion. The pins or posts so spaced produce no observable adverseeffect on the main forward beam, the primary effect of their presencebeyond the desired one of substantially eliminating sidelobes of greatlytroublesome magnitude, particularly close to the main lobe, being aslight change of impedance which is readily accommodated by theimpedance-matching construction which must in any event be used, where,as is common, the exciter probe is itself excited from a stationarywaveguide.

In the construction to be described, the posts or pins just mentionedare formed in such a way as to serve the additional function ofstrengthening the entire structure mechanically and providing accuratespacing across the vertical dimension of the cavity, the rigidity andaccuracy of spacing produced at the mouth cooperating with the rigidparabolic back wall to make the entire cavity interior completely stabledimensionally as compared with the stability and dimensional uniformitywhich can be maintained with the same thickness of material of theparallel walls of the cavity without the reinforcement and spacingfunction provided by the pins or posts across the mouth.

Although the invention is of particular utility in the pill-box type ofconstruction, of which an-embodiment is illustrated in the drawing anddescribed below in accordance with the provisions of the patent laws,certain of the features will be adaptable to use with antennas ofsomewhat different construction, as will be observed by persons skilledin the art. The invention will best be understood in both its narrowerand broader aspects from the description of the embodiment illustratedin the draw ing annexed, in which:

FIGURE 1 is a top plan view, partially broken away in section, of anantenna constructed in accordance with the invention;

FIGURE 2 is a vertical sectional view taken along the line 2-2 of FIGURE1 in the direction indicated by arrows, the hub portion employed forrotation of the antenna being shown in elevation;

FIGURE 3 is a front elevational view of the antenna;

FIGURE 4 is a fragmentary enlarged sectional view showing the details ofa post or pin construction employed in the illustrated embodiment; and

FIGURE 5 is a comparative pattern plot illustrating the improvement inpattern characteristics obtained by the improvement of the invention.

In the illustrated embodiment, the pill-box cavity 10 is formed by a topplate 12 and a bottom plate 14 connected by a back wall 16 extendingperpendicular to the parallel plates 12 and 14 and shaped in the form ofa plane parabola having its focus at the intersection of the parabolicaxis and the line connecting the opposite edges of the parabola. Theparabola as illustrated is symmetrical with respect to the axis, as iscommon in antennas of this type. The top and bottom walls or plates 12and 14 are carried slightly forward of the parabolic portion of thecavity, and side plates 18, parallel with each other and with theparabolic axis, cooperate with this forward extension of the top andbottom plates to form an open mouth 20 projecting from the parabolicportion, the plates 12 and 14 being bent respectively upward anddownward from this point out to form horn flare portions 22 and 24, withfurther bends forming flanges 26, the sidewalls 18 being formed withappropriate taper of vertical dimensions to produce the enclosed hornform commonly used to produce the desired vertical plane pattern andimpedance transformation of the coupling from the interior to theexterior of the antenna cavity.

The feed, generally designated by the numeral 28, has an excitingconductor 30 extending vertically in the cavity at the focus point ofthe parabola, this conductor being the protruding center conductor of arigid coaxial line 32 which is fixedly mounted at its bottom end (notshown), where it is coupled by a suitable transistion and impedancematch to the waveguide which is conventionally employed in such systems.The region of the cavity exterior surrounding the coaxial line 32 isprovided with a hub 34 mounted on suitable bearings isolatedelectrically from the cavity by the usual grooving, etc, employed insuch rotating joints (not shown) and is provided with a drive gear 36for rotation.

The exciting conductor or probe 30 is thus on the rotational axis of theantenna so that its orientation relative to all parts of the rotatingcavity 10 remains constant throughout. Respectively disposed rearwardand forward of the probe 30 are a director 38 and reflector 40consisting of screw-posts of the relative lengths and spacings from theprobe which are normally used in such directive elements.

Arranged in a straight line across the entire mouth 20 of the cavity arepins or posts 42. The horizontal dimensions of the cavity 10 are verylarge relative to the wavelength at the frequency of operation, theparabolic aperture or the width of the mouth 20 thus being a largenumber of wavelengths. The pins or posts 42 are formed by conductingtubes 44 secured by rivets 46 extending through suitable apertures inthe top and bottom plates 12 and 14.

The transverse dimension of the pin or post structures 42 is so selectedas to be very small compared to a wavelength. However, the diameter ofround conductors such as here illustrated is preferably sutiicient toperform the mechanical support and spacing function mentioned, althoughthin wires can of course be used in instances where this aspect of theadvantages of the construction can be sacrificed.

To meet the requirements that the conductors 42 must not, by theirdimensions and spacing, interfere substantially with main lobe or beamtransmission, but must provide substantially complete shielding ofdirect radiations from the probe 30 at angles at which the reflector 40is ineffective, it is of course possible to use more complex forms ofconstruction such as thin plates. However, by proper selection ofspacing and pin diameter, the same effect is produced with roundconductors without the complexity of fabrication and assembly requiredfor the use of plates or vanes, and with simple installation in a mannerassuring proper cavity dimensioning. A spacing between adjacentconductors of at least a half wavelength and at most slightly more thana wavelength is satisfactory, but reduction of the openings between pinsto less than a half wavelength impairs the entire operation. As earlierindicated, the pin diameter should be less than a quarter wavelength;with the latter limitation, center-t0- center pin spacings greater thanone-and-one-half wavelengths, corresponding to openings much wider thana wavelength, commence to seriously impair the sidelobe reductioncharacteristics achieved by the construction illustrated. Where roundpins or posts are used for producing rigidity and accuracy of cavitydimensions, a diameter of about one-eighth wavelength is desirable, thepreferred dimensioning employing pins of from two to three sixteenthswavelength diameter spaced by somewhat less than one wavelength.

In addition to the shielding function just discussed, the pinconstruction serves an important purpose in providing rigidity anddimensional stability. With the use of the tubes 44-, secured byexternal fasteners such as the illustrated rivets, or bolts, the heightat the mouth is accurately controlled by the tube length, thus giving adimensional stability and constancy across the width which is otherwiseunobtainable in any comparably simple fashion.

One construction of the illustrated device was designed for operation ata frequency of 9.4 gc. An aperture of approximately 22 inches wasemployed with pins (tubes) of three-sixteenths inch outer diameter withequal center-to-center spacing of inches between the central and each ofthe outermost of the ten pins illustrated on each side thereof in thedrawing, other constructional features being more or less conventional(reflector onequarter wave forward of the radiator probe, etc). The lineof pins was located slightly less than threequarters of an inch forwardof the probe. The forward pattern in the horizontal plane obtained atthis frequency is shown in FIGURE 5, the dotted curve indicating theresults obtained before installation of the pins, and the solid curveshowing the pattern as obtained with the pin construction justdescribed. n this graph or plot, as is conventional, the maximum of theradiation is shown as the 0 db level, with the other ordinate valuesbeing related thereto as a function of angle with respect to the axis.It will be ob served that the improvement effected by the addition ofthe pins is extremely great. The first sidelobe (at about seven or eightde rees from the main beam peak) is reduced by almost three db, whileall further sidelobe structure is reduced by ten db or more, theimprovement in the entire sidelobe structure outside of the firstsidelobe region being extreme, as will be observed.

it will of course be understood that the theory underlying theexperimental finding of vast improvement typified by 5, as discussedabove, cannot be easily experimentally verified. The extent to which theimprovement is obtained by the shielding effect, and the extent to whichit is obtained by the exactness of dimensioning produced at the mouth,cannot readily be determined. it is believed that it is the combinedeffects that produce the remarkable improvement experimentally found,but the protection to be afforded the invention should not be impairedby lack of present knowledge as to the exact theory of operation of theconstruction experimentally found to be advantageous, nor should theprotection be limited to the particular embodiment illustrated in thedrawing and described above, many variants being readily obvious, andothers being observable by those skilled in the art after study. Thestructure of the invention is described in the appended claims.

What is claimed is:

It. in a directive antenna having:

(a) a radiator cavity having a pair of plane parallel conducting wallsand a rear wall extending perpendicularly to, and connecting, saidparallel walls, the rear Wall being in the general form of a parabolahaving its focus in the front portion of the cavity, the front portionof the cavity being substantially open and forming the radiator mouth,and

(b) a feed in the cavity at the focus of the parabolic back wallcomprising a substantially omnidirectional radiating probe element and areflector conductor adjacently forward thereof to impede direct externalradiation from the feed in the directly forward direction, the improvedconstruction having:

(c) parallel mutually spaced conductors of rearward-toforward dimensionsmaller than the spacing extending perpendicularly between andconnecting the parallel walls at the open mouth portion of the cavityslightly forward of the feed to impede direct external radiation fromthe feed in the laterally forward directions.

2. The antenna of claim 1 wherein said spaced conductors are round pins.

3. The antenna of claim 1 wherein said spaced conductors are equallyspaced in a straight line across the entire mouth, the spacing betweenadjacent conductors being at least a half wavelength and at mostslightly more than a wavelength.

4. The antenna of claim 3 wherein said spaced conductors are round andof less than a quarter wavelength diameter.

5. The antenna of claim 1 wherein the parallel Walls are formed by metalplates, said spaced conductors being of equal length and having theadjacent portions of the parallel Walls supported and spaced thereby tohold the spacing of the walls rigidly uniform.

6. The antenna of claim 5 wherein the conductors are metal tubes, inregister with apertures in the parallel plates and secured by fastenersextending through the apertures and plates to clamp the inner surfacesof the plates against the ends of the tubes.

7. In a directive antenna having:

(a) a radiator cavity having a pair of plane parallel conducting wallsand a rear wall extending perpendicularly to, and connecting, saidparallel walls, the rear Wall being in the general form of a parabolahaving focus in the front portion of the cavity, the front portion ofthe cavity being substantially open and forming the radiator mouth, and

(b) a feed in the cavity at the focus of the parabolic back wallcomprising a substantially omindirectional radiating probe element and areflector conductor adjacently forward thereof to impede direct externalradiation from the feed in the directly forward direction, the improvedconstruction having:

(c) conductors dispersed along the mouth and connecting the parallelwalls and of a spacing and shape producing substantially completeshielding of the exterior from direct radiations from the feed in thelarge-angle forward directions in which the reflecting means is leastefiective while having substantially no effect on the parallel forwardtransmissions from the parabolic wall.

8. The antenna of claim 7 having a horn portion forward of the mouth ofthe cavity, the horn portion having side-webs shielded from the directradiation from the feed by the conductors.

9. The antenna of claim '7 wherein the latter conductors are round pinsof a diameter less than a quarter wavelength and of equal spacingbetween a half and one-and-one half wavelengths, and said conductorsbeing in a straight line extending across the mouth of the cavity, theperpendicular distance from the feed to the line defined by theconductors being substantially less than a wavelength.

iii. in a directive antenna having:

(a) a parabolic reflecting element, and

(b) a feed at the focus of the parabolic element coming substantiallyless than a quarter wavelength and prising a radiating conductor ofessentially nonthe conductors being arranged in a straight line, theperdirectional radiating characteristics in the plane perpendiculaf di tf th f d t th li d fi d by pendicular to its direction Of orientationand a re the conductors being 1355 than a Wavelengflm flector conductoradjacently forward of the radiating 5 element to reflect radiationdirectly from the ele- References Cited by the Examiner ment in theforward dlrection, the improved con- UNITED STATES PATENTS structioncomprlsing:

(c) conductors dispersed across the front of the para- 2,393,095 4/1946KatZin 343 786 bolic element parallel with the radiating conductor 102,415,807 2/ 1947 BarfOW et a1 343909 and forward thereof and mutuallyspaced in the direc- 2,591,486 4/ 1952 Wilkinson 343-786 tionperpendicular to their extension and of a spac- 2,724,054 11/1955 Yevick343-780 ing and shape producing substantia ly comp ete 2,743,440 4/1956Riblet 343-783 shielding of direct radiations from the feed in the 2 7 350 9 195 ()rtusi 343 7 3 large-angle forward directions in which thel'efie t- 15 2 764,757 9/1955 Rust 343 7g3 ing means is least effectivewhile having substantial- 2,825 062 2/1958 Chu et all 343 780 1y noeffect on the parallel forward transmis ions 2884629 4/1959 Mas)I1 343730 from the parabolic element.

11. The antenna of claim 10 wherein the latter con- ELI LIEBERMAN ActingPrimary Examiner ductors are round with equal spacings of between one-20 half and one-and-one half wavelength, the diameter be- MA AR A CH,Exammer.

1. IN A DIRECTIVE ANTENNA HAVING: (A) A RADIATOR CAVITY HAVING A PAIR OFPLANE PARALLEL CONDUCTING WALLS AND A REAR WALL EXTENDINGPERPENDICULARLY TO, A CONNECTING, SAID PARALLEL WALLS, THE REAR WALLBEING IN THE GENERAL FORM OF A PARABOLA HAVING ITS FOCUS IN THE FRONTPORTION OF THE CAVITY THE FRONT PORTION OF THE CAVITY BEINGSUBSTANTIALLY OPEN AND FORMING THE RADIATOR MOUTH, AND (B) A FEED IN THECAVITY AT THE FOCUS OF THE PARABOLIC BACK WALL COMPRISING ASUBSTANTIALLY OMNIDIRECTIONAL RADIATING PROBE ELEMENT AND A REFLECTORCONDUCTOR ADJACENTLY FORWARD THEREOF TO IMPEDE DIRECT EXTERNAL RADIATIONFROM THE FEED IN THE DIRECTLY FORWARD DIRECTION, THE IMPROVEDCONSTRUCTION HAVING: (C) PARALLEL MUTUALLY SPACED CONDUCTORS OFREARWARD-TOFORWARD DIMENSION SMALLER THAN THE SPACING EXTENDINGPERPENDICULARLY BETWEEN ANC CONNECTING THE PARALLEL WALLS AT THE OPENMOUTH PORTION OF THE CAVITY SLIGHTLY FORWARD OF THE FEED TO IMPEDEDIRECT EXTERNAL RADIATION FROM THE FEED IN THE LATERALLY FORWARDDIRECTIONS.